It is required to coat the surface of electric contact portions of electronic parts with a noble metal having a high corrosion resistance and outstanding electrical characteristics. At present, while gold electrodeposition is chiefly adopted to coat the surface of electric contact portions, electrodeposition methods encounter difficulties in forming deposits of uniform thickness on electronic parts of finely elaborate and complicated shape.
On the other hand, electroless plating methods can deposit a coating of uniform thickness even on electronic parts of finely elaborate and intricate shape. Consequently it is desired to develop methods for treating the surface of such parts with noble metals by electroless plating. Of platinum group metals, palladium is the most inexpensive and is expected to find extensive industrial applications.
A typical electroless palladium plating solution heretofore known is an aqueous solution containing a bivalent palladium salt as a metal source, ammonia as a complexing agent, ethylenediaminetetraacetic acid or a salt thereof as a stabilizing agent and hydrazine as a reducing agent.
This plating solution, however, has the serious drawback of being unpreservable because of its poor stability and susceptibility to spontaneous decomposition. Further said plating solution has the defect of being quickly decomposable due to the Pd delivered from a pretreating solution. Such plating solution containing hydrazine as a reducing agent suffers the additional shortcoming that when a substrate has been immersed in the plating solution for a longer period of time, the depression rate is pronouncedly reduced although only small amounts of effective components in the plating solution have been consumed. Moreover, the plating solution poses the problem of requiring ammonia in a high concentration as a complexing agent because of the low stability of the plating solution, hence undesirable in terms of work environment.
Also known is an electroless palladium plating solution comprising a bivalent palladium salt, ethylenediaminetetraacetate, ethylenediamine and sodium hypophosphite (Japanese Examined Patent Publication No. 26764/1971). Yet this plating solution is defective in having a low stability and decomposing in a short period of time.
None of the plating solutions described above can be applied to electronic parts because they deposit a coating having numerous cracks and being poor in solderability. These plating solutions entail further drawbacks such that in deposition of thick coating, the deposition rate is noticeably decreased with the increase of thickness of coating being deposited, resulting in coating which is blackened and thus impaired in appearance.
Accordingly the electroless palladium plating solutions heretofore reported can be said to remain on a laboratory scale presently.
Pd-Ni alloy coatings obtained by alloying nickel with palladium are known to be unlikely even in an atmosphere of organic gas to have an undesired effect attributable to the presence of organic gas such as the contact failure in an electric contact portion caused by the polymerization of organic gas. While it is consequently desired to put Pd-Ni alloy plating to practical use, few reports have been made on electroless Pd-Ni alloy plating solutions usable with an industrial feasibility.